The present invention relates to a lifting system for lifting interchangeable truck bodies or freight containers adapted to be hauled by automotive trucks.
Such truck bodies or containers are known which are provided at their four corners with hydraulic cylinders having vertical axes. These cylinders serve to make the body or container interchangeable, permitting its removal from a hauling vehicle during loading and unloading operations, and its subsequent mounting on the same or on another vehicle for transport purposes. Furthermore, during the loading and unloading operations, these cylinders enable the height of the bottom of the body or container to be continuously varied, thereby facilitating the operations.
Known lifting devices have a shortcoming concerned with the synchronization of the lift cylinders, that is to say, the operation of the four cylinders at the same moment, even with a load badly centered in the body or freight container. If in fact the four cylinders are operated by a single pump and if a badly centered load is placed in the body, it may happen that the pump fluid will pass more easily into the less loaded cylinders, thereby unequalizing the system and possibly overturning the body or container.
In order to overcome this fault, it has been proposed in the art to feed the four cylinders by means of flow dividers, which are able to supply the same amounts of fluid to the four cylinders independently of the inlet pressures of the latter.
Nevertheless, this solution has various shortcomings. Firstly, the same extension of the four cylinders insures that the body or container will rest on the ground only if the latter is entirely flat. Secondly, the limited precision of the flow dividers, due to the length of the cylinders, may give rise to an unacceptable difference in the extension of the cylinder, since the points of support on the ground may not be coplanar. Finally, the hydraulic circuit is fairly complex, delicate, and expensive.
Therefore, according to the present invention, the shortcomings of the known prior art are completely corrected by a novel fluid circuit arrangement in which the four lift cylinders are incorporated. In a preferred embodiment of the invention, the truck body or container lifting system comprises four doubleacting cylinders which are respectively disposed at the four corners of a rectangular array, the cylinders comprising two pairs of cylinders, each pair of which includes one cylinder which is connected in series to the cylinder at the diagonally opposite corner of the rectangular array.
Each cylinder of the system is preferably vertically disposed and contains a piston which divides the cylinder into an upper and a lower chamber, each pair of cylinders comprising a cylinder whose lower chamber is connected to the upper chamber of the other cylinder of the pair. Preferably, the effective cross sectional areas of the interconnected upper and lower chambers are equal to each other.
Means may be provided for insuring that the fluid pressures prevailing in the two said pairs are substantially equal. Thus, the means may comprise maximum pressure relief valves which are connected between an inlet and an outlet of one of the two cylinders of each pair.
Preferably, the upper chamber of one cylinder of each said pair is connected to a common first pipe for the supply of pressure fluid thereto, the lower chamber of one cylinder of each pair being connected to a common second pipe for the supply of pressure fluid thereto.
The upper ends of the cylinders may engage a common load.
Other features and advantages of the invention will become apparent during the course of the following description.